Some acoustic insulation tasks are best performed with a combination of a primary, relatively thick fibrous sheet and a secondary, thinner sheet or membrane (the term “membrane” means herein a thin sheet). In combination, the primary sheet and membrane reduce noise better than the primary sheet would by itself; for example, inclusion of the membrane can often enhance reduction of noise in lower frequency ranges. In addition, the membrane can offer physical protection for the primary sheet.
Membranes can also be used by themselves, mounted in a planar array over an air space (i.e., with the film stretched in a flat or curved plane over the air space or air gap). When positioned in a room or other enclosure in which it is desired to reduce noise, with an air gap of appropriate thickness behind the membrane, the membrane functions to absorb sounds in the enclosure.
A composite insulation of a thick primary insulating sheet and a secondary membrane-like sheet is described, for example, in Thorn et al., U.S. Pat. No. 6,376, 396, which teaches as the membrane-like sheet a nonwoven web compacted in two sequential operations—a first stage of mechanical compaction (such as by needle-tacking or hydroentangling) and a second stage of compaction by heat and pressure with presses or calenders. Increased sound absorption is said to occur as a result of the second compaction.
Another prior art teaching is Tilton, U.S. Patent Application Publication No. US 2004/0002274 A1, which teaches building panels, ceiling tiles, or similar building components in which a decorative fabric previously used to cover the exterior of the building component is replaced by a densified facing layer of polyester fibers. The densified facing layer, which is printed with aesthetically pleasing graphics or other indicia, is said to reduce costs and also to provide improved insulating properties.
See also Vanbemmel et al., U. S. Pat. No. 6,720,068, which focuses on a multilayered product or laminate useful as sound-absorbing insulation for automobiles. Recognizing a need for the insulation to be cheap, thin and lightweight, the patent describes a product comprising a backing layer—an open-cell foam or a sparingly compacted nonwoven fibrous web—coated with an extremely thin layer of microfibers (col. 2, 11. 7-9). The coating of microfibers “attains a thickness of only 0.2 to 1.0 mm, and in particular 0.3 to 0.7 mm and has a weight per unit area of 20 to 200 g/m2, and in particular 30 to 100 g/m2”(col. 1, 11. 62-64). There is no specific example or similar description in the patent of a laminate of the Vanbemmel invention.
Another commercial form of insulation uses a membrane-like sheet that comprises a multilayer sheet such as a spunbond-meltblown-spunbond laminate (SMS) point bonded with heat and pressure and assembled in combination with a primary insulating sheet.
A disadvantage with each of the described prior insulation composites is that in order to add desired levels of sound insulation to the composite the membrane-like secondary sheet is thick enough and heavy enough to add undesired cost and weight to the overall insulation package.